Impacts of elevated N inputs on north temperate forest soil C storage, C/N, and net N-mineralization

Nitrogen (N) availability influences carbon (C) storage in forest soils through effects on plant growth, litter production, and soil C decomposition and stabilization. Soil C/N and net N-mineralization are commonly reported empirical measures of soil N availability, and changes in these variables due to N inputs may feed back to soil C storage through a variety of biotic and abiotic pathways. We conducted a meta-analysis of the responses of soil C storage, C/N, and net N-mineralization (Nmin) to N inputs in north temperate forests. The modes of N addition we considered were the establishment of N-fixing vegetation (Nfix), N-fertilization (Nfert), and simulated chronic N deposition (Ndep). Overall, N inputs increased soil C (+ 7.7%) and Nmin (+ 62%), while decreasing C/N (− 4.9%). Soil C concentrations and C storage (pool sizes) responded similarly to N inputs, although increased soil C storage occurred only in the mineral soil (+ 12.2%). C/N shifts were restricted to the forest floor (− 7.8%), whereas forest floor and mineral soil Nmin responses to N inputs were very similar (+ 61% and + 64%, respectively). Significant between-study variation in the effects of N inputs on soil parameters was related to the mode of N addition, biogeographic factors, and time. Temporal relationships between N inputs and soil response parameters suggested that increases in forest floor and mineral soil Nmin were large but transient, and were followed by decadal-scale changes in forest floor C/N and mineral soil C storage. Temporal patterns present in the dataset could have been due to changes in soil organic matter decomposition and stabilization, or increased plant productivity and litter inputs. In either case, the results of this study quantitatively demonstrate that managing forest N supply affects N availability and soil C storage, and highlight the importance of long-term measurements for accurately assessing the effects of N inputs on forest soils.